There are already known several pedicle screws for the fixation of the vertebral column having the advantage that the different pedicle screws can be implanted independently from the longitudinal support. The longitudinal support is subsequently entered into the pedicle screw either sidewardly or from the top and fastened. For example, such a pedicle screw is disclosed in EP-B 0 330 881 SHERMAN.
The disadvantage of these known pedicle screws consists in the relatively complicated locking mechanism to fasten the longitudinal support after inserting it into the openly shaped head of the pedicle screw. Furthermore, in most cases the head of the screw is only adjustable with respect to the position of the longitudinal support to a limited degree, thereby requiring complicated and time-consuming adjustment of the longitudinal support.
DE-C 195 09 332 discloses an anchoring element in which the spherical head of a pedicle screw can be locked into a compressible pressure element at various angles. A tensioning of the pressure element in the retainer head is generated due to a mechanism previously disclosed in PCT/CH92/00125, i.e., by moving the truncated cone shaped pressure element in a complementorily conically shaped bore until the pressure element jams in the bore hole. By jamming the pressure element, the spherical head of the bone screw can be secured at various angles.
The disadvantages of these known devices include the following:
a) The design of the hollow cylindrical pressure element which accepts the spherical head provides only two downwardly open longitudinal slots extending in the same direction (in the direction of the bone screw). Therefore, while jamming no homogenous encompassment of the spherical head of the bone screw via the lamellas of the slotted pressure element takes place. This inhomogenous encompassment of the spherical head causes a reduced jam force; PA0 b) The bone screw must be mounted by inserting it together with the pressure element from the top via the recess for the tensioning element and fastened secured against rotation in the device. This means the bone screw cannot be screwed in independently from the retainer head. The retainer head enables no mounting after the implantation of the bone screw. As a result a reduced view while screwing in the bone screw with mounted retainer head is yielded. If a system with differently formed retainer heads should be offered, the system consists, because of the premounting of the bone screw caused by the design, of n*m implants [where n=number of head variants and m=number of lengths of the screws] instead of n+m implants. A further disadvantage is that a change of the retainer heads in situ caused by the situation is impossible. PA0 c) The low stability of the angle, which permits only a fixation by tension boom; PA0 d) Through the tension means, the longitudinal support of the device and the spherical head of the bone screw are fastened at the same time. A loosening of the tension means causes a failure of the whole fixation system at once; and PA0 e) Depending on the situation, one would like to fasten the connection between the device and the spherical head of the bone screw independently from the longitudinal support to distract or compress between longitudinal support and bone screw via the longitudinal support without loss of angle. PA0 easy handling PA0 inserting of screw part of the device without the retainer head; PA0 possibility of subsequent onclicking of the retainer head; PA0 depending on the situation or the preference of the surgeon free choice of or mixture of sidewardly open, upwardly open or closed retainer heads. For example an upwardly open retainer head eases the insert of the longitudinal support while a sideward opening enables correction sidewardly; and PA0 depending on the embodiment separate fastening of the longitudinal support and spherical head of the bone screw.